If the ADSL modem has only a single ethernet port, chances are it doesn't have a built-in router and it expects to be connected to a single MAC device. Using a simple switch would not work in such a circumstance because a switch doesn't have a MAC address and the model would receive call from multiple computer's MAC; you would need a router for that.
That's not true. Zyxel 660 DSL modems have a single etherenet but have built-in routers.
Good to know. Here in Quebec, all major ISP provide the modem and none of them support routing, they are simply gateways. To help the OP, maybe people could contribute a list of DSL modems that have built-in router. After that the OP will be able to decide which solution he prefers (modem/router + switch vs modem+switch).